1. Field of the Invention
The present invention relates to a method of making a chip resistor for surface mounting on a printed circuit board for example.
2. Description of the Related Art
Recently, for enhancing a mounting density on a circuit board, various electronic components are being replaced with chip-type components which can be surface-mounted. As a typical example of chip-type electronic component, there exists a chip resistor of the type as shown in FIG. 26. Specifically, the chip resistor includes a substrate 70 made of a ceramic material for example, film electrodes 71 formed to cover the opposite side surfaces and some parts of the upper and the lower surfaces of the substrate 70, a film resistor 72 bridging the film electrodes 71 on the upper surface of the substrate 70, and a protective coating 73 for protecting the film resistor 72.
The chip resistor is made generally in the following manner. Specifically, as shown in FIG. 27, use is made of a mother substrate 74 which is substantially flat and prepared by baking a ceramic material. The mother substrate 74 is formed with a plurality of longitudinally-dividing grooves 75 (hereinafter referred to as BB slit which is an abbreviation of bar break slit) arranged at a regular pitch, and a plurality of transversely-dividing grooves 76 (hereinafter referred to as CB slit which is an abbreviation of chip break slit) arranged at a regular pitch. The respective slits 75, 76 define generally rectangular unit substrates 77 which are finally formed into chip resistors.
Subsequently, as shown in FIG. 28, film electrodes 71 as electrode terminals are collectively formed on the upper surface of the mother substrate 74. Specifically, the film electrodes are formed at opposite ends of each of the unit substrates 77 by printing and baking. Thereafter, film resistors 72 are collectively formed on the unit substrates 77 by printing and baking.
The mother substrate 74 is then divided widthwise along the BB slits 75 to provide intermediate products each in the form of a narrow strip. Subsequently, after predetermined electrode material is printed and baked on the side and the lower surfaces of each of the intermediate products, the intermediate product is divided along the CB slits 76.
The resistance of each of the film resistors 72 is adjusted by so-called laser trimming in the state of the mother substrate 74. Specifically, a trimming groove is formed on each film resistor 72 by laser application for providing a predetermined resistance while measuring the resistance with a measurement probe brought into contact with film electrodes 71 provided at opposite ends of the film resistor 72.
In forming the film electrodes 71 and the film resistors 72 by the above-described chip resistor making, a print mask 79 as shown in FIG. 29, which includes openings 78 formed in accordance with the printing pattern of the film electrodes 71 or the like, is disposed on the mother substrate 74. In this state, by moving a squeegee on the print mask 79 for example, printing paste is applied and printed on the mother substrate 74 through the openings 78 of the print mask 79.
However, when the mother substrate 74 made of ceramic material is baked, the mother substrate 74 may shrink widthwise or longitudinally to some extent. Therefore, when the above-described print mask 79 is disposed on the mother substrate 74, the pitch of the openings 78 of the print mask 79 does not coincide with the pitch of the BB slits 75 and CB slits 76, thereby causing print deviation.
Hitherto, therefore, a plurality of (practically no less than 100 kinds of) print masks 79 are prepared which are identical in printing pattern but slightly different in position of openings 78. Thus, for applying printing paste on the mother substrate 74 to form film electrodes 71 or the like, a print mask 79 having openings 78 which correspond to the slits 75, 76 of the shrunk mother substrate 74 is selected and disposed on the mother substrate 74. However, preparation of a multiplicity of print masks 79 in accordance with shrinkage of the mother substrate 74 is uneconomical.
Further, in recent years, size-reduction of a chip resistor is increasingly demanded. However, there is a limitation on and a difficulty in reducing the size of a chip resistor by the above-described method which utilizes the print mask 79 for forming the film electrodes 71 and the film resistors 72.
On the other hand, JP-A-63-224305 discloses another method of making a chip resistor. This method comprises the steps of extruding a substrate material into a non-baked green substrate bar, printing electrodes and resistors on the green substrate bar, simultaneously baking the substrate bar together with the electrodes and the resistors, and finally dividing the substrate bar into unit substrates.
With this method, since the substrate bar together with the electrodes and the resistors are simultaneously baked, it is possible to alleviate the printing deviation of the electrodes and the resistors relative to the substrate bar, or the printing deviation of the resistors relative to the electrodes.
However, the above-described method includes the step of forming, by extrusion, a non-baked green substrate having a cross section which corresponds to the cross section of an aimed chip resistor. With this method, however, it is very difficult to form extremely small chip resistors of a dimension of, for example, no more than 1 mm so as to have uniform cross sections. This is partially because it is very difficult to adjust a substrate material, which is a suspension of ceramic particles and a solvent so as to have viscosity suitable for extrusion. Further, it is not practical to form a green substrate bar of a uniform cross section by continuously extruding the substrate material through a very small nozzle hole.
It is therefore an object of the present invention to provide a method of making a chip resistor which is capable of realizing size reduction of a chip resistor and enhancing production efficiency.
In accordance with the present invention, there is provided a method of making chip resistors each of which comprises a unit substrate which is rectangular as viewed in plan and has a predetermined thickness, a resistor element provided on an upper surface of the substrate, and electrodes provided at opposite ends of the unit substrate, the method comprising the steps of:
continuously forming a green sheet;
obtaining from the green sheet an intermediate product in the form of a narrow strip on which electrodes and resistor elements are printed, at least the resistor elements being printed at a pitch corresponding to the unit substrates;
forming slits on the intermediate product for dividing the intermediate product into the unit substrates, each of the slits extending perpendicularly to the longitudinal direction of the intermediate product;
simultaneously baking the intermediate product together with the printed electrodes and the printed resistor elements; and
dividing the baked intermediate product along the slits into the unit substrates.
In a preferred embodiment, the step of obtaining the intermediate product is performed by cutting the green sheet into a narrow substrate strip of a predetermined width extending longitudinally of the green sheet followed by printing thereon the electrodes and the resistor elements.
In a preferred embodiment, corners of the substrate strip are rounded in cutting the green sheet into the narrow substrate strip of the predetermined width.
In a preferred embodiment, an upper surface of the narrow substrate strip is formed, at widthwise opposite edges thereof, with longitudinally extending stepped portions of a predetermined width.
In a preferred embodiment, an upper surface of the narrow substrate strip is formed, at a widthwise central portion thereof, with a longitudinally extending recess having a predetermined width.
In a preferred embodiment, in the step of obtaining the intermediate product, the electrodes and the resistor elements are printed by longitudinally transferring the narrow substrate strip and rotating printing rollers in contact with an upper surface, side surfaces and lower surface of the narrow substrate strip.
In a preferred embodiment, in the step of obtaining the intermediate product, the narrow substrate strip is longitudinally transferred while the electrodes and the resistor elements are printed by performing inkjet printing with respect to an upper surface, side surfaces and lower surface of the narrow substrate strip.
In a preferred embodiment, in the step of forming the slits on the intermediate product, each of the slits is formed to have a depth which is about one half of a thickness of the intermediate product.
In a preferred embodiment, the step of dividing the baked intermediate product along the slits into the unit substrates is performed by attaching the baked intermediate product onto a stretchable tape for transferring while tightly sandwiching the same between dividing rollers provided above and below.
Preferably, in this case, the method further includes the step of performing resistance adjustment after the division into the unit substrates, and the resistance adjustment is performed while conducting resistance measurement for each of the resistor elements by bringing electrode rollers into contact with corresponding electrodes of each unit substrate with the stretchable tape stretched.
In a preferred embodiment, each of the resistor elements is subjected to resistance adjustment with respect to the intermediate product after the baking and before the division into unit substrates.
Preferably, in this case, the resistance adjustment for each of the resistor elements is performed while performing resistance measurement for the resistor element by bringing electrode rollers into contact with corresponding electrodes on the intermediate product.
In a preferred embodiment, the step of obtaining the intermediate product is performed by cutting the green sheet into the substrate strip of a predetermined width after printing a plurality of electrodes and resistor elements on an upper surface of the green sheet in a matrix arrangement.
Preferably, in this case, the printing is performed using a print mask which includes openings corresponding to an electrode pattern or a resistor element pattern.